In order to maximize a system performance and a transmission capacity in the field of wireless communications, a multiple input multiple output (MIMO) system has attracted attentions. The MIMO system can enhance transmission efficiency by employing multiple transmitting antennas and multiple receiving antennas. When the number of antennas in the transmitter and receiver increases simultaneously, the theoretical channel transmission capacity increases in proportion to the number of antennas. It is possible to markedly enhance spectral efficiency.
The MIMO system has attracted attentions as a next-generation technology capable of overcoming a restriction on the data rate which has reached a limit due to an increase in data communication in the field of mobile communications. Since the next-generation mobile communications require a higher data rate than that of the existing mobile communications, efficient multiple antenna technology is needed. The MIMO technology is one of the next-generation mobile communication technologies which can be widely used.
The MIMO system collects fragmentary data pieces received by a plurality of antennas to complete data transmission. It is possible to enhance a data rate in a specific range or to enhance the system coverage at a specific data rate.
Various MIMO techniques have been actively studied from various viewpoints such as information theory associated with system capacity, measuring and modeling a wireless channel and a space-time signal processing for enhancing transmission reliability and data rate.
The MIMO techniques are roughly classified into a spatial diversity and a spatial multiplexing. The spatial diversity allows a multiple antenna to transmit the same data. The spatial diversity can enhance the transmission reliability by transmitting the same data through multi-path. The spatial diversity is divided into a space-time block coding (STBC) and a space-time trellis coding (STTC). The STBC has an advantage that complexity is low. The STTC has an advantage that the performance for enhancing a bit error rate (BER) and a degree of freedom for generating codes is high. By using the spatial diversity, it is possible to obtain a gain corresponding to a product of the number of transmitting antennas and the number of receiving antennas.
The spatial multiplexing allows the multiple antenna to transmit data different from each other. The receiver removes the interference by the use of a proper signal processing technique and then reproduces the data. Conventional interference removing techniques are a maximum likelihood method, a zero forcing (ZF) method, a minimum mean-square error (MMSE) method, a diagonal-bell laboratory layered space-time (D-BLAST) method, a vertical-BLAST (V-BLAST) method, etc. When channel information is known, singular value decomposition (SVD) may be used.
The spatial diversity gain can be saturated as diversity order increases. The spatial multiplexing can deteriorate the transmission reliability of wireless channels. The spatial diversity and the spatial multiplexing can be combined to acquire gains of both techniques.
A fading channel is widely known as a main reason to deteriorate performance of a wireless communication system. A channel in fading environment may vary depending on time, frequency, and space. To overcome the fading, a diversity technique uses the fact that the probability that all independent channels have low channel gains is very low.
A multi-user diversity is one of various diversity techniques. When a plurality of users exists in a cell, the channel gains of the plurality of users are independent of each other in probability. The probability that all the users have low channel gains is very low. The multi-user diversity can be classified into a time multi-user diversity, a frequency multi-user diversity, and a spatial multi-user diversity.
In the time multi-user diversity, the channel is assigned to the user having the highest channel gain when the channel varies with the lapse of time. According to the information theory, when transmission power is enough, the total capacity of the channels can be maximized by assigning all the channels to the user having the highest channel gain. In the frequency multi-user diversity, a sub-carrier is assigned to the user having the highest channel gain in a frequency multiplexing system. The spatial multi-user diversity uses the channel gains of the user varying depending on spaces. An example of the spatial multi-user diversity is a random beam-forming (RBF). The RBF is also called an opportunistic beam-forming. The RBF induces a variation of the channel by allowing a transmitter to perform a beam-forming process with a weighting value.
A closed loop structure is providing a feedback channel from a user equipment to a base station for enhancing the performance of the multiple antenna system. The close loops structure can be used in the spatial diversity or the spatial multiplexing. Information transmitted from the user equipment to the base station through the feedback channel is channel information. When the user equipment transmits the channel information to the base station, the base station can maximize the performance by controlling various system parameters such as power level and transmission format. An adaptive modulation and coding (AMC) technique enhances link performance by allowing the base station to control the modulation and coding scheme (MCS) based on the channel information. The AMC technique supports efficient transmission by increasing the data rate when the channel is good and decreasing the data rate when the channel is faded.
As the channel information becomes closer to the actual channel, it is more advantageous to enhance system performance, increase transmission efficiency and remove interference.
There is a need for a technique of representing channel information to be closer to the actual channel and transmitting the channel information more efficiently.